Chart showing the eurypterid localities of New York. The dots give the approximate number of species occurring in each locality; their size indicates the relative frequency of the species. The belt ruled from right to left denotes the exposure of the Salina beds; that ruled from left to right the exposure of the Shawangunk grit. The eurypterid-bearing area of the Frankfort beds is inclosed between the dotted lines.

IIIGEOLOGICAL DISTRIBUTION AND BIONOMIC RELATIONS

In this chapter we shall first survey the geological distribution of the eurypterids in North America as indicated by the following conspectus, compare this distribution with that in Europe, and finally attempt a conclusion as to the physical conditions under which these strange creatures lived.

AConspectus of American species arranged according to their geological occurrence

It is not necessary for us to discuss here the Algonkian[3] and Cambric occurrences, further than to mention that the Cambric species which is a true eurypterid, conclusively demonstrates the great age of the merostomes and their early existence in truly marine beds.[4]

The earliest appearance of the eurypterids in profusion is in the Lower Siluric Frankfort shale of Schenectady and Schoharie counties, N. Y., lately discovered. Here at least 11 species, at present referred to the genera Eurypterus, Dolichopterus, Stylonurus, Eusarcus, Hughmilleria and Pteryrgotus, have been found to range through the eastern littoral marine development of the formation.

The earliest appearance of the characteristic biologic facies of the Eurypterida in America is that in the waterlime of Kokomo, Indiana which contains four species of the order. Kindle [1904] has distinguished two stratigraphic horizons in the Niagaran of Indiana, which correspond to the Lockport and Guelph formations of New York respectively, and Schuchert [1904] is of the opinion that, judging from the associated brachiopods, the Kokomo cement beds are probably of Noblesville, i.e. essentially Lockport age, and surely not of the age of the Bertie waterlime of New York, since no beds younger than the Guelph are known from northern Indiana. The aspect of the Kokomo fauna is in full accordance with this correlation since at least two of the species (Onychopterus kokomoensis and Drepanopterus longicaudatus) may be considered as older types than the species of the Bertie waterlime and following faunas.

The Guelph dolomite, like the Clinton, has afforded only a single straggler, the eurypterid facies of the horizon not yet having been observed; but the directly following age, that of the basal Salina, is represented by two faunas in New York State, viz, those of the Pittsford and Shawangunk shales. While these two have no species in common, they are characterized as probably belonging to approximate horizons by the presence of the genus Hughmilleria in both and by the similarity of their sedimentary and faunal aspects in general.

The Pittsford Eurypterus bed has been found by Sarle to be but 20 feet from the base of the Salina group; while the Shawangunk grit rests unconformably on the upturned edges of the Lower Siluric shale and, before the discovery of its eurypterid fauna, had been referred to the Salina by Hartnagel [1903, p. 1175; 1907, p. 50] on purely stratigraphical evidence, the latter consisting in the fact that the Shawangunk grit is conformably overlain by a series of formations of upper Salina age.

The Pittsford shale is separated by the main body of the Salina formation (Vernon and Camillus shales) from the principal eurypterid-bearing horizon of the State, the Bertie waterlime. In the exposures of the latter about Jerusalem hill in Herkimer county and in the quarries at Buffalo it has afforded the fauna described by Hall in the Palaeontology of New York and later exploited by Grote, Pitt and Pohlman. It is a fauna in which the genera Eurypterus and Pterygotus prevail in number of species (Eurypterus with five, Pterygotus with three) and in the size of the creatures, which are there obviously near the climax of their development. It is also the horizon where individual development is greatest.[5]

One or two stragglers, notably Eurypterus remipes, return in the Rondout waterlime in greatly diminished number, and a new peculiar type of Eurypterus (E. microphthalmus) appears in waterlime intercalations of the Manlius limestone above.[6]

The North American Devonic has furnished but scanty remains of Pterygotus in the Dalhousie formation of New Brunswick,[7] the Gaspé sandstone of lower Quebec,[8] the fish beds at Campbellton, N. B.,[9] the Stylonurus? wrightianus of the Portage sandstone, the gigantic S. excelsior of the Catskill beds of New York and S. beecheri from the Chemung of Pennsylvania.

The Waverly beds of Pennsylvania, near the New York boundary, have furnished a single straggler in E. approximatus but the Productive Coal Measures of Pennsylvania frequently contain remains of the peculiar phylogerontic group of the genus Eurypterus, distinguished as Anthraconectes.

CGeological distribution in other countries

In Scotland and on the shores of the Baltic occur beds comparable in wealth of merostomes with those of New York, and there the eurypterid horizons exhibit a remarkable parallelism with our series and are approximately homotaxial.

The lowest distinct eurypterid horizon in Scotland is the lower one in Lanarkshire and the Pentland hills, characterized by species of Eurypterus and Stylonurus and especially by the genus Slimonia and is now correlated with the Wenlock.[10] It hence corresponds in age to our lowest American eurypterid horizon, that of the Kokomo waterlime. It has in common with the latter the primitive stylonuroid genus Drepanopterus.

The upper horizon of the Lanarkshire eurypterids is in beds that protrude, islandlike, from the Old Red sandstone and for this reason were formerly confused with the latter but are now correlated with the Ludlow and the "Passage" beds. Their fauna corresponds in age to our Salina faunas. The presence of species properly referred to Eusarcus (Eurypterus scorpioides Woodward) and Hughmilleria (Eurypterus lanceolatus Salter) is faunistic evidence of this homotaxy.

Still closer is the faunistic and stratigraphic agreement of the Bertie waterlime with the eurypterid beds of Oesel. The upper Oesel zone contains in Eurypterus fischeri a species which is conceded by Schmidt and Holm to be but a vicarious form of the E. remipes; and as both the Bertie waterlime and the upper Oesel are situated close to the top of the Siluric, there is little doubt that they are homotaxial [Schmidt, 1892].

The Devonic of Great Britain contains one eurypterid horizon, that of the Old Red sandstone, characterized by the giant Pterygotus anglicus, the "seraphim" of the Scottish quarrymen. To this monster of the great lakes and estuaries of the Old Red sandstone continent, Eria, the Stylonurus excelsior of the Catskill beds is a parallel; it lived at the same time, under like physical conditions, in the same continental waters and is found in a similar association of ganoids (Holoptychius), placoderms and land plants. In a similar association these remains are found in the Upper Devonic of Belgium and of New Brunswick.

Minor occurrences are also known from Podolia, Galicia, and Bohemia [Barrande, 1872; Semper, 1898; Seeman, 1906]. Those of Podolia and Galicia are essentially a continuation of the Oesel horizon [v. Alth, 1874; v. Siemiradzki, 1906]. Australia has furnished a single fragment of Pterygotus from the Upper Siluric [McCoy, 1899].

The last outburst occurs both in Great Britain and North America in the Productive Coal Measures where a number of species of Eurypterus appear which bear the distinct marks of the approaching extinction of the race, and because of their phylogerontic characters these have been united under the subgenus Anthraconectes. In North America the Coal Measures of Pennsylvania and the iron stone nodules of Mazon Creek, Illinois, have furnished about half a dozen species of this peculiar group.[11]

The Permic of Portugal has furnished the final straggler of the race in a small Eurypterus [de Lima, 1890].

DBionomy of the eurypterid faunas

A very interesting problem in the study of the eurypterids is that of their bionomic relations and geologic facies. A philosophic contribution to the discussion of this problem has been given by Professor Chamberlin in his paper On the Habitat of the Early Vertebrates [1900, p. 400].

Chamberlin's hypothesis is to the effect that "the fish and the eurypterids descended from the rivers to the sea in the mid-Paleozoic, though their remote ancestors may have ascended from it," and the principal argument in its support is found in the claim that "there is only one conspicuous type that is facilely suited to free life, independent of the bottom, in swift streams, and that is the fish form"; it is further urged that "this could have developed only in water that possesses a persistent and usually rather rapid motion in a fixed direction, i. e., in rivers."

In support of this hypothesis it is pointed out that the Paleozoic fish and the similarly built eurypterids are always associated and it is suggested that the two possessed a parallel development due to the same physical influence. This view of the fresh-water origin of the eurypterids is directly contradictory to the current view among paleontologists of their originally marine habitat and later adoption of first a brackish and finally a freshwater life. Zittel-Eastman's Textbook of Paleontology expresses this prevailing view as follows [1896, p. 673]:

They are found associated with graptolites, cephalopods, and trilobites in the Ordovician of Bohemia and North America; with marine Crustacea (Phyllocarids and Ostracods) in the Silurian; with Ostracoderms and Arthrodires in the Devonian; and with land plants, scorpions, insects, fishes and fresh-water amphibians in the Productive Coal Measures. It is apparent, therefore, that from being originally marine forms, they became gradually adapted to brackish, and possibly even fresh-water conditions.

In describing the geologic occurrence of the fish and eurypterids, Chamberlin lays special stress on the abrupt appearance of both groups of fossils in the Siluric, heralded by very scanty remains in the preceding formations, and gives the following account of the phenomena of their distribution in the rocks.

In the Ludlow "bone bed" of England, where they first make their appearance in abundance, the fish remains are associated with eurypterids, probably the most gigantic crustaceans that have ever lived, some of them attaining two meters in length. There is the same association on the continent, notably in the island of Oesel in the Russian Baltic and in Podolia and Galicia, and so again in the waterlime group of America in which the Pteraspis [Palaeaspis] americana of Claypole occurs. The physical conditions in all these cases seem to have been peculiar, and in the case of the waterlime group they were singularly so, for they permitted a host of these larger eurypterids and other crustaceans to flourish in seeming luxuriance, while only a meager and pauperate marine fauna found an occasional entrance into the series. The conditions seem to have been congenial to the fish and eurypterids but not to a typical marine fauna.

In the Old Red sandstone of the Devonian both in Europe and America a similar association obtained. A most extraordinary group of fishes and a family of most gigantic crustaceans flourished where marine life found only an occasional and meager presence. These few marine forms, here and there in a massive deposit, no more imply prevalent salt water than the present marine species in the bay of San Francisco imply that the gravels, sands and silts of the valley of California and of the Great Basin, which seem to be analogues of the Old Red sandstone, are prevailingly marine. The further association of the fishes and eurypterids with land plants and fresh-water mollusks, together with a total absence of marine relics from the same beds, leaves no solid ground for hesitating to accept the dominant view of English and other geologists that the typical Old Red sandstone and its homologues are the deposits of fresh waters and that both the fishes and the eurypterids found congenial conditions of life in them. As fishes and eurypterids were found both earlier and later in marine deposits the question arises: Were the fishes and eurypterids primarily marine and later became adapted to fresh water, or were they primarily fresh-water forms which were occasionally carried out to sea, and which later became adapted to salt water? The two cases do not necessarily require an identical answer, but the singular association of the two in unusual display under peculiar conditions and on both continents strongly implies a community of habit, at least at the stages in question. The association is one of the most unique [!] faunal and physical combinations of geologic history.

The earlier occurrence of the eurypterids in marine deposits is almost as limited as that of the fishes, and yet they were well adapted to fossilization and were actually fossilized as far back as Precambrian times, as Walcott has recently shown by their discovery in the Belt Mountain terrane of Montana. Of about a dozen known genera of eurypterids, only two or three of those least well known are without associations with formations regarded as fresh water. The relics found in marine sediments may be attributed to transportation from the land just as is done in the case of the terrestrial plants and land insects not infrequently found in marine beds; but transportation in the opposite direction can not be assigned . . . From the occurrence of eurypterids first in marine beds apparently and later in fresh-water deposits it has been inferred that they were originally sea dwellers and later became adapted to land waters, but the meagerness of their marine record on the one hand, and their abundance and fine preservation in the fresh-water deposits on the other give point to the question whether their early marine record is anything more than the chance deposit of river forms borne out to sea.

In view of the contrasting opinions which have been thus expressed as to the original habitat of the eurypterids, it will be well to analyze closely here the evidence from the rocks which contain the eurypterids, and from the associated species.

In regard to the Cambric Strabops thacheri, the Lower Siluric Echinognathus clevelandi and Megalograptus welchi, the Clinton Eurypterus prominens and the Guelph E. boylei, we might concede, in view of the fact that all these remains have been found in only a single individual each, that they are remains carried out to sea from terrestrial waters, yet their combined evidence inclines to the side of the marine habitat.

The profuse Lower Siluric (Frankfort) fauna is associated with seaweeds, graptolites, trilobites, cephalopods and brachiopods, and inhabited the pools of a littoral region with abundant detrital sediments. All the Lower Siluric eurypterids thus far known were hence still purely marine.[12]

The rich faunas of the Kokomo and Salina beds (the latter containing these creatures by thousands) are all intercalated in distinctly marine deposits; the Kokomo beds carry such brachiopods as Conchidium colletti and Wilsonia kokomoensis, and the Salina eurypterid shales Leperditias, Pterineas (P. subplana), cephalopods (Orthoceras, Gomphoceras), marine gastropods, Conularias, Lingulas and Orbiculoideas. The same is true of the European eurypterid horizons intercalated in the Wenlock and Ludlow beds of Great Britain, and of the Oesel beds of Russia. The latter horizon lies between the lower Oesel zone with heavy coral banks, numerous trilobites (as Calymmene blumenbachi, Encrinurus punctatus, etc.) and brachiopods (Orthis elegantula, etc.) and the so called Ostergarn beds which still contain Eurypterus, but also Lucina (Ilionia) prisca, Meristina didyma, Leperditia and in their uppermost layers species of Chonetes, Spirifer, Beyrichia, etc.

Thus the Eurypterus beds of the Salina formation in which the fauna reaches its climacteric development, are clearly stamped with their marine origin, and the profusion and perfection of preservation of the eurypterid remains precludes the possibility of their transportation into the basin by land waters; it is also apparent that the beds were not formed under normal marine conditions. The eurypterid horizons of Kokomo, of the Salina and of Oesel, as well as of Great Britain, exhibit as clearly all the characteristics of a particular and peculiar marine facies, as do the graptolites or the corals. These facies are indicated partly by the unusual nature of the rocks and partly by the peculiar aspect of the associated animals. The most characteristic of eurypterid rocks is the waterlime.[13] It is in this that the faunas of Kokomo and of the Bertie waterlime of New York are contained. How close is this association of eurypterids and waterlime, is shown by the fact that with the recurrence of waterlime formation (Rondout) in the Salina section after the close of the salt deposition and the Cobleskill dolomitic limestone stage, Eurypterus reappears, though much diminished in number, and later again, after the introduction of the Manlius fauna the upper waterlime bed of the Manlius brings back a single species (E. microphthalmus). The section at Manlius given by Hartnagel[14] [1903, p. 1165] illustrates well the intimate connection of waterlimes with eurypterids on one hand and the intercalation of limestones with Stromatopora and brachiopods on the other.

The peculiar yellow plattenkalke and marls of Oesel which contain the well known Baltic eurypterid fauna are of similar character with the Salina eurypterid beds.

A different rock facies containing eurypterids is represented in the Pittsford shale and the Shawangunk grit shale beds. In the former the fossils occur in a rather fissile, very dark olive-green to black shale. Its relations to the Salina section have been recorded by Sarle [1903, p. 1082].[15]

These sections show that the Eurypterus-bearing black shales are intercalated in red and green shales with thinner beds of dolomites and mudrocks. The waterlime and shale occurrences of Eurypteri in these Salina beds are hence not expressions of different biologic facies. It is very different with the Eurypterus horizon in the Shawangunk grit. There the euryptericls occur in thin shale intercalations that are found in endless repetition between heavy beds of conglomerate and grit.[16]Connected with this difference in rock is that in the aspect of the fauna. While the Eurypterus faunas of Buffalo and Pittsford consist almost entirely of mature and adolescent individuals, the younger growth stages prevail in the Shawangunk grit shales and large individuals are represented only by fragments which indicate that they may have been destroyed by the more turbulent water conditions farther out. On the other hand, the general absence of the earlier growth stages at Buffalo and Pittsford, together with the presence of the finer grained rock, intimate that there we have the habitat of the mature eurypterids which probably, was the somewhat deeper littoral, as in the case of Limulus, while the shale intercalations of the Shawangunk grit are the deposits of the shore pools in which the larvae were hatched and where the earliest stages of the ontogeny were passed, again as in the living Limulus.

An interesting analogy between this breeding place of the eurypterids in Orange county and that of Limulus rests in the fact that the young of Limulus are hatched on sandy tide flats and tide zones and the Shawangunk grit is a coarse sand deposit of this kind. It seems, therefore, a proper conclusion that the Shawangunk grit represents a tidal zone deposit of an encroaching sea or of a delta.

Omitting for the present the peculiar case of the Eurypterus horizons in the Frankfort shale and the Shawangunk grit and returning to the more typical Eurypterus facies represented by the waterlimes and shales of the Salina formation of western New York, we encounter the venerable and still current view that these beds were formed under conditions of much abnormal salinity. This view is based on the presence of the salt and gypsum deposits of the Salina formation and the absence of fossils in the middle Salina beds (Vernon, Camillus shale) which were early ascribed to "salt pan" conditions in the Salina sea. The authors have pointed out in Memoir 5 [Guelph Fauna of New York, p. 117] that the preceding Guelph was a distinct phase in the development of the vast Niagara coral sea into the desiccating, more or less inclosed sea of the Salina stage, the fauna of the Guelph already exhibiting characters suggestive of the increased salinity of the sea. The cycle of events leading to the culmination and decline of the Salina sea has been expressed by Hartnagel in the appended diagram and described as follows:

DECREASING
SALINITY

A
B

Niagara-Guelph
fauna

Cobleskill

C

Waterlime
with Eurypterus

Salina

D

Gypseous
shales

Beds of
rock salt

INCREASING
SALINITY

D

Gypseous
shales

C

Pittsford shale
with Eurypterus

B

Guelph
fauna

Niagaran

A

Niagara
fauna

With the ever increasing salinity of the waters the Guelph fauna retreats, and next in the black Pittsford shale at the base of the Salina there occur Eurypteri, and with them constantly associated a species of Lingula. With the retreat of this fauna we find, as physical changes went on, deposits of gypseous shale and later the salt beds. The deposition of these great beds of rock salt marks the turning point in this cycle, . . . beds of gypsum were again deposited, but never again were the conditions favorable for the deposition of extensive beds of rock salt. Following the gypsum beds, we have the Salina waterlime with its splendid Eurypterus fauna, and associated with the Eurypterus is a species of Lingula similar to the one at the base of the Salina. Above the Eurypterus beds follows the Cobleskill limestone, and here again are representatives of the Niagara-Guelph fauna.

If the cycle just described is considered as produced by the desiccation of a closed basin, in which extreme salt pan conditions prevailed at its climax when the thick beds of rock salt were deposited, it follows as a manifest corollary that the eurypterids of Salina age had their biologic optimum in a sea of greater salinity than the typical mollusks and trilobites of the Upper Siluric could endure.

In view of the hypotheses before us and the evidence that the eurypterids flourished in brackish and fresh water in the Devonic and Carbonic this corollary requires a closer study.

The English geologists, notably Hugh Miller and Lyell, in the middle of the last century, explained the origin of salt deposits by the evaporation of sea water in basins so separated from the ocean by shallow bars that the evaporated water could be replaced by new marine water while the corresponding more saline water could not flow out on the bottom. This bar theory has been based on safe physical and chemical data by Ochsenius,[17] while von Koenen[18] and others have shown that this theory on the whole explains the complex composition of the German salt deposits.

A close analysis of the Salina sections and of the character of the Salina rocks also suggests this conception as fully competent to explain the conditions surrounding their deposition.

The facts which we consider as of especial importance to a correct view of the physical conditions of the Salina sea are: (1) the continuous alternations of gypseous and dolomite beds, (2) the great thickness of the salt beds.

The oft repeated alternation of gypseous beds with shales or limestone is illustrated by the section at Buffalo[19], where five to six repetitions of gypsum and shale are exposed. A like repetition of gypsum beds, the gypsum mostly in thin seams and nodular layers, is shown in the section of the Livonia salt shaft,[20] Livingston county, N. Y. This alternation of the gypseous beds with the dolomites and shales indicates a constant change of conditions which is difficult of explanation in a slowly and regularly desiccating basin, but denotes a periodic interruption of the drying process, either by inflow over the bar, or, perhaps, by seasonal freshets.

Another feature of the Salina beds favoring the bar theory is the great thickness of the salt beds. In the Retsof salt shaft at York, Livingston co., [Luther, op. cit. p. 118] the drill passed successively through 22 feet of salt, 30 feet of shale and limestone and again through 58 feet of salt.[21] Recent writers[22] have held that such thicknesses of salt can not be explained by the desiccation of a sea enclosed by land barriers in distinction from bars. The salt of the Retsof mine would require the desiccation of a sea 1750 feet deep, and this is irreconcilable with the shallow water conditions of the Salina beds evidenced by the frequent sun cracks in the dolomites and waterlimes of the formation.

There is no doubt that the culminant salt pan condition of the Salina period with its heavy precipitation of salt, implies an arid climate. There is also evidence indicating the persistence of these desert conditions throughout the Salina period and at the time when the eurypterid-bearing sediments were deposited. This evidence is found in (1) the scarcity of carbonaceous matter in the Salina beds, (2) the prevalence of dolomites and waterlimes.

The Salina beds are notably free from carbonaceous matter when compared with the underlying Niagaran and overlying Helderbergian beds. This may be partly due to the absence of such decaying marine organisms as furnished the bitumen with which part of the underlying Guelph dolomite is saturated, but it is also an indication of the absence of vegetation on the adjoining land. This becomes especially manifest if it is considered that the Salina sea was almost entirely surrounded by land, and that, as the frequent sun cracks in the waterlimes of central New York demonstrate, the shore was nowhere very distant. Several authors, as Joh. Walther [1900], Joseph Barrell [Geol. Jour., 1908] have, in recent years, pointed out the absence of carbonaceous matter in littoral and delta deposits as characteristic of an arid climate.

While thus the salt and gypsum-bearing deposits serve to demonstrate the increase of land-locking and salinity of the lagoon and the subarid to arid climatic conditions, the black and green Pittsford shales indicate that then the aridity had not reached its climax and that the lagoon or estuary still received at certain seasons both clastic sediments and fresh water.

It is therefore hardly necessary to infer that the eurypterids of the Salina period lived in a brine. It is quite possible that, when the lagoon became too saline, they withdrew into the brackish water zone of estuaries or deltas. This inference agrees well with the meagerness of the marine brachiopod and mollusk fauna with which they are associated, the brackish water being still today the least inhabited zone of the hydrosphere on account of its frequent changes in salinity; and it is also in full accord with the occurrence of the eurypterids in the Devonic rocks. There is no longer any doubt that the Stylonurus of the Catskill beds inhabited an estuary with brackish water conditions; and in regard to the Old Red sandstones which are currently considered by the British geologists as fresh-water deposits, Kayser in his excellent textbook has noted [p. 168] that the presence of whole layers of brachiopods and other genuine marine shells in the Old Red sandstone of St Petersburg proves the prevalence, at least temporarily, of brackish water lagoon conditions, arid Clarke[23] has described the lagoons indicated by the Upper Devonic deposits of eastern New York, comparing these with the conditions prevailing now in the bar-locked lagoons of the Prussian Baltic with their shifting of freshwater and brackish faunas. The eurypterid beds of Lanarkshire and the Pentland hills, of Ludlow age, are regarded by British geologists as brackish water deposits, for the reason that they contain eurypterids, phyllocarids, limulids, scorpions and myriapods together with fish and land plants. It therefore seems proper to conclude that the eurypterids in Siluric time were prevailingly inhabitants of the brackish water zone, and in Devonic time they were wholly so.

In the Carbonic era on this continent the eurypterids are mainly perpetuated by the peculiar subgenus Anthraconectes, in the Productive Coal Measures. These are found in Pennsylvania on slabs densely covered with fern leaves and other plant remains which can not fail to suggest near land and probable fresh-water conditions. Yet even in this case, the fragmentary condition of many of the fern leaves which would, according to the criteria of the "Allochthonie" of the coal measures, advanced by Potonié[24] indicate the transported condition of the material; even more the profuse presence of the small spiral tubes of the marine Spirorbis on the plant remains, as shown by Simpson's excellent figure [Hall, 1884, pl. 6] afford a caution against an unqualified conclusion as to the fresh-water habit of these Carbonic eurypterids. In a recent exhaustive discussion, Girty[25] has reached the conclusion that it seems most reasonable to regard the fauna of the Carbonic of Pennsylvania "as a natural assemblage of species selected and modified by a habitat, if not in strictly marine, at least not in strictly fresh waters."

There is, however, clear evidence at hand of the fresh-water habitat of the Carbonic eurypterids in other regions. One of these is the occurrence of Eurypterus (Adelophthalmus) granosus Jordan in the coal measures of Saarbrücken. That basin was formed in the interior of a continent and never reached by the sea.[26] In other coal basins, notably those of England[27] the gradual freshening of the lagoons and the disappearance of such marine types as Lingula and Orbiculoidea, which are frequent in the lower coal measures, has been clearly recognized. It is only in the upper measures that the eurypterids occur, there in association with ostracods, phyllopods and schizopod crustaceans. Woodward [1879, p. 198] makes a positive statement that the Carbonic Eurypterus scouleri is a fresh-water form, basing his conclusion on the character of the rocks and the associated flora and fauna.

There seems hence little doubt that the eurypterids of the Carbonic finally abandoned the sea and entered the fresh water. Directly thereafter, in the Permic they became extinct, the last of the race being the Eurypterus douvillei de Lima, found in the Rotliegende of Portugal associated with Walchia pinniformis and Sphenophyllum thorni.

Summarizing these data we conclude that the eurypterids lived in the sea from Cambric to Siluric time. They had then become less sensitive to changes, positive and negative, in the salinity of the water. In fact they seem to have thrived best under conditions of life that exclude most other marine groups of animals, that is, in the marginal, more or less inclosed marine lagoons, accompanied by estuaries receiving delta-forming terrestrial drainage, with prevailing arid or subarid climate, the waters being in some places more than normally briny, in others having less than normal salinity. In other words they were euryhaline or able to live in both salt and brackish water.

Their adaptation to such conditions is paralleled today by such crustaceans as Apus and Artemia which not only thrive under rapid diminution of normal salinity but, by means of strongly protected eggs, even survive salt pan conditions which end in complete desiccation, as shown by their well known occurrence in desert lakes. The usual associates of the Siluric eurypterids are peculiar crustaceans whose nature emphasizes the reference above made. They are phyllocarids and ostracods and members of the strange family Hemiaspidae (Neolimulus, Bunodes, Hemiaspis, Pseudoniscus). This congeries of peculiar crustaceans seems to constitute a fauna especially adapted to, and therefore highly characteristic of, lagoon and estuary conditions.

Thus while the earlier eurypterids were marine and their climacteric fauna euryhaline; their later habit throughout the Devonic and Carbonic led them finally into the fresh water.

The succession of habitats is hence, according to our evidence, the reverse of that suggested by Chamberlin's hypothesis noted at the beginning of this discussion.[28]

The cause of the withdrawal from the sea of these well armed and often gigantic eurypterids into the brackish and fresh water is a problem of much interest. Perhaps the development of the more agile and more advanced fishes put these slow and archaic merostomes on the defensive and finally forced them altogether out of the sea. Their association with clumsy and heavily armed, equally archaic Old Red fishes which clearly suffered a like fate from their own more advanced relatives, would seem to be very suggestive in this connection.

It may be mentioned that even the gigantism of these arachnids, as typified by Stylonurus excelsior, is probably an indication of race degeneracy, as gigantism is generally, and as such is also suggestive of their increasing failure to cope with the conditions of marine life.

↑There occur gigantic tracks in the Potsdam rocks of New York which, have been considered by good authorities as suggesting the presence of merostomes at that age. These tracks known as Climactichnites, were first described by Logan [Can. Nat. & Geol. 1860. v. 5] and later recorded by Hall [N. Y. State Mus. 42d Rep't. 1889. p. 25] from Port Henry, Essex co., N. Y., and by Woodworth [N. Y. State Mus. Bul. 69. 1903. p. 959] from the town of Mooers, Clinton co., N. Y. In the latter locality they assume gigantic proportions, being 6 inches wide and 15 or more feet long, terminating in an oval impression 16 inches long.
Various explanations have been suggested for these tracks. Besides having been referred to trilobites, burrowing crustaceans, plants, gastropods and annelids, they have been compared with those of the horseshoe crab, first by Dawson and recently again by Hitchcock and Patten. Sir William Dawson [Can. Nat. & Geol. 1862. 7: 271], who studied the American Limulus on the seashore, pointed out that when Limulus creeps on quicksand, or on sand just covered with water it uses its ordinary walking legs and produces a track strikingly like that described as Protichnites from the Potsdam sandstone, but in shallow water just covering the body, it uses its abdominal gill plates and produces a ladderlike track the exact counterpart of the Climactichnites except that in the track of Limulus the lateral and medial lines are furrows instead of ridges. Patten [Science. n.s. 1908. 28: 382] "described the movements of a modern Limulus in advancing up a sandy beach with the tide and the action of the abdominal gill plates making rhythmic ridges in the sand. He compared these with the tracks of Climactichnites which he ascribed to forms related to the eurypterids rather than the trilobites. The tracks showed a beginning in a hollow in the sand and where continued on the specimen to the further end there became fainter, as if the animal rose from the bottom. This would correspond with the habit of the Limulus, which remains buried on recession of the tide and upon its first return crawls and then swims away. Beside one track were seen two symmetrically placed impressions attributed to the longer arms of a Eurypteroid form."
In favor of this view is the fact that Strabops is a Cambric eurypterid that would appear competent to produce such tracks; but Woodworth has brought forward arguments to the effect that the trail was made by a mollusk and the sedentary impression is the end of the trail [op. cit. p. 961, 964] instead of its beginning. The direction of the obliquely transverse marks of Climactichnites is always toward the oval impressions and comparison with those of the Limulus tracks [Dawson, fig. 1-3, and also fig. 157 in Cambridge Nat. Hist. v. 4] would indicate that the animal, if an eurypterid, moved toward the sedentary impression and not away from it.

↑Since this memoir went to press information has been received of new occurrences of eurypterids in the Upper Siluric of Pennsylvania and Maryland which we have been able to investigate by the favor of Dr Ulrich and Mr Jesse E. Hyde.
One specimen, collected by Mr Billingsley at the Delaware Water Gap, is a slab of black shale of the Shawangunk grit with indeterminable patches suggestive but not demonstrative of eurypterid integument. (SeeAppendix).
From the National Museum are three lots. The first, two split pieces of one slab of waterlime, is from the Salina of Selinsgrove Junction, Pa., and exhibits segments which suggest an Eurypterus, like E. remipes. This occurrence was recorded by Schuchert in 1903 [Lower Devonic and Ontaric Formations of Maryland, p. 416]. In character of the rock and the association of these remains with large Leperditias this slab is very like the Bertie waterlime.
The second lot consists of "black shale interbedded in the Keefer sandstone member of the McKenzie formation (basal Cayugan), Lock 53, 4 or 5 miles above Hancock, Md." The shale and the mode of preservation of the fossils here are as at Otisville. The material contains some recognizable parts of the integument; two carapaces, some tergites, small patches with ornamentation, and a telson. All these, save the carapaces, have the characteristics of a Pterygotus, especially in the sculpture which consists of large, semicircular, posteriorly rising scales and the telson. The smaller of the two carapaces is either a distorted Dolichopterus comparable to D. otisius, or a Pterygotus, approaching a Slimonia in outline. The larger is too incomplete for determination; what there is of it also points to the Hughmilleria-Pterygotus group. There is also a small fragment that suggests a badly crumpled carapace of Hughmilleria. On account of the interest attaching to this new locality, we have figured the carapaces and telson [pl. 70, fig. 6–8].
The third lot consists of thin slabs of waterlime, collected, according to the label, from the upper part of the McKenzie formation. In this the remains of the integument are so comminuted, that but a small tergite and several pieces with Pterygotus sculpture are recognizable.
It appears from these small lots of fossils that the peculiar eurypterid facies, both of the waterlime and of the shale extended in Cayugan time into the narrow bay reaching southward from New York into the Appalachian basin. Favorable conditions, such as for a time existed at Otisville through extensive quarrying, will undoubtedly some day bring these faunas more fully to light.

↑Though we have been discussing the eurypterids of the Bertie waterlime as if they constituted a single congeries, there is actually a very marked geographic distinction in their distribution. At the two productive localities, Erie county and Herkimer county, there are noteworthy distinctions in respect to species. The two faunules are as follows:

HERKIMER COUNTY

ERIE COUNTY

Eurypterus remipes

Eurypterus lacustris

Dolichopterus macrochirus

E. lacustris var. pachychirus

D. testudineus

E. pustulosus

Pterygotus macrophthalmus

Eusarcus scorpionis

P. cobbi

Dolichopterus macrochirus

D. siluriceps

Pterygotus buffaloensis

P. cobbi

P. grandis

The species common to both are Dolichopterus macrochirus and Pterygotus cobbi, both of which are quite rare, while the predominant species in both places are unlike. It is not believed that these differences necessarily express distinct stratigraphic horizons, as both congeries lie near the top of the waterlime succession, but rather indicate original regional separation into distinct lagoons or pools, so that we may without impropriety speak of these regions as the Buffalo pool and the Herkimer pool, which we may assume to have been synchronous. There is, in the face of the differences suggested, a certain degree of approximation in the two expressed by such vicarious species as E. remipes and lacustris, P. macrophthalmus and buffaloensis, which may well mean distinctions due to geographic isolation. The Herkimer pool is well restricted and its faunule can not be traced very far toward the west; the Buffalo E. lacustris, however, appears alone as far east as Union Springs, Cayuga co., and as far west as Bertie, Ontario. Another difference in these faunas is the preponderating great size of all the species in the Buffalo pool and, by contrast, the small size of and abundant young among the Herkimer county species; a distinction which may be due to differences in depth. That the smaller creatures lived in conditions of shallower water is evinced by the sun-dried and cracked rock surfaces of their matrix, while such evidences are wanting in the Buffalo pool; and indeed it would be quite in accordance with our acquaintance with the limulids to find the smaller and younger forms in the shallower shore waters, where there might be laid down with their remains the drifted fragments of larger individuals such as are indicated by the great Pterygotus segment and the large carapace of E. remipes, elsewhere figured. While E. remipes seems to have strayed farthest beyond the bounds of its lagoon, yet both pools were really quite restricted and today the most productive part of the Buffalo pool seems to have been removed by the quarrying operations of the Buffalo Cement Company.

↑To show the more exact position of the Eurypterus-bearing beds in the entire succession of the Siluric, this briefer tabulation is appended, in which a descending order is followed. The productive beds are in italics.

devonic

Helderbergian

Manlius

Rondout

Cobleskill

Bertie

Camillus

Salina

Salt

Upper Siluric

Vernon

Pittsford-Shawangunk

Guelph

Niagaran

Lockport-Noblesville

Rochester

Clinton

Medina-Richmond

Lower Siluric

Frankfort

Utica

↑Clarke, N. Y. State Mus. Mem. 9,2: 18, 1909. The diminutive Eurypterus? pulicaris Salter and Eurypterella ornata Matthew from the Devonic rocks of New Brunswick are so little known and their eurypterid nature so doubtful that they are here left out of consideration.

↑The investigation of this Frankfort shale fauna proves it to be restricted geographically to the exposures of the formation in the lower Mohawk valley, where it seems to pass with equal profusion through a thickness of several thousand feet. Specimens have been obtained in the Dettbarn quarry at Schenectady, the bluestone quarries at Aqueduct and Rexford Flats, on both sides of the Mohawk river; and a profuse association of Sphenothallus and the eurypterids was found in the upper part of the section described by Cumings [N. Y. State Mus. Bul. 34, p. 451] from Rotterdam Junction, west of Schenectady. In much higher beds of the Frankfort shale the same fauna was obtained in abundance at quarries at Duanesburg and Delanson, on the Schenectady branch of the Delaware & Hudson Railroad, and still farther southwest in the reentrant of the Helderberg escarpment, caused by the Schoharie creek, in exposures along that creek above Schoharie Junction. In a ravine of a small stream which joins the Cobleskill between Central Bridge and Howes Cave (mentioned by Grabau [N. Y. State Mus. Bul. 92, p. 102] on account of its fine exposure of the Brayman shale), the eurypterid fauna could be traced quite up to the Brayman shale. Thus in the western part of its range it occupies the whole thickness of the Frankfort shale, unless it is absent in the lowermost part of the formation, not yet found in good exposures in that region. The fauna does not seem to extend to the western localities, for it has not been observed in the Frankfort shale sections at Frankfort and Ilion, where the fossils have been very thoroughly studied by the junior author.
Toward the southeast the last traces of the eurypterids are found in the sandy shales exposed along the Vly, below Voorheesville. Here the eurypterid beds are followed after an interval by several hundred feet of shales and sandstones that form the top of the Frankfort formation and contain a distinct fauna without eurypterids. These rocks, elsewhere designated the Indian Ladder beds, are typically exposed in a high bluff below the Indian Ladder at the Helderberg escarpment.
The peculiar restriction of the eurypterids to the easternmost exposures of the Frankfort shale would seem at first to be explicable, as in the case of the Salina faunas, by assuming their occurrence in "pools." But the Frankfort shale exhibits notable differences from the Salina in total thickness, faunal association and lithological character as between the eastern and western occurrences, evidence which tends to indicate that the eastern beds were deposited close to the shore line, and the more western beds farther offshore. The Frankfort eurypterids were thus inhabitants of the shallow littoral waters with their mud flats and lagoons, and if the shore line of the late Utica-Frankfort period extended in northeast-southwest direction, just east of the lower Mohawk region, as conceived by Schuchert [Paleogeography of North America, pl. 60] the exposures along the lower Mohawk valley happen to intersect only their former narrow habitat. The range of the eurypterids extends in this limited area through a thickness of from 1500 to 2000 feet which is due to the fact that this region was apparently involved in the Appalachian folding then going on, and thereby suffered depression while large quantities of material were swept down from the rising land in the east.

↑Another consideration which antagonizes that hypothesis is the fact that the brackish fauna, small as it is, as a rule is composed of species which entered the brackish zone from the sea and not by such as descended from the fresh-water lakes and the rivers. This has been shown by Walther [Einleitung in die Geologie als historische Wissenschaft. 1894. 1 Theil. Bionomie des Meeres] to be especially true of the mollusks and crustaceans.